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Ultraviolet (UV) Disinfection in Water Treatment Hans van Leeuwen. Department of Civil, Construction and Environmental Engineering Iowa State University April 15, 2011

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Ancient Hindu source written at least 4000 years ago - raw water be boiled, exposed to sunlight, filtered, and then cooled in an earthen vessel. Germicidal properties of sunlight: 1887 Artificial UV light (Mercury lamp) developed: 1901 First application in drinking water: Marseilles, France in 1910 Substantial research on UV in the first half of 20 th century History of UV Disinfection Limited field application: Low cost and maturity of Cl 2 disinfection technology coupled with operation problems associated with early UV systems

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Mechanisms of UV Disinfection Disinfection by UV radiation- physical process- electromagnetic waves are transferred from a UV source to an organisms cellular materials (especially genetic materials) UV light does not necessarily kill the microbial cell UV light inactivates microorganisms by damaging nucleic acids (DNA or RNA) thereby interfering with replication of the microorganisms and therefore incapable of infecting a host Different microorganisms have different degree of susceptibility to UV radiation depending on DNA content Viruses are the most resistant Microbial repair: regain of infectivity

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UV Dose The effectiveness of UV disinfection is based on the UV The effectiveness of UV disinfection is based on the UV dose to which the microorganisms are exposed dose to which the microorganisms are exposed UV dose is analogous to Cl 2 dose UV dose is analogous to Cl 2 dose Cl 2 dose = Cl 2 conc. x contact time (t) or Cx t Cl 2 dose = Cl 2 conc. x contact time (t) or Cx t UV dose (D) = I x t or if intensity not constant UV dose (D) = I x t or if intensity not constant Where, D = UV dose, mW.s/cm 2 or mJ/cm 2 I = UV intensity, mW/cm 2 I = UV intensity, mW/cm 2 t = exposure time, s t = exposure time, s UV dose can be varied by varying either the intensity or the UV dose can be varied by varying either the intensity or the contact time contact time

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UV Disinfection Kinetics – Similar to Cl 2 Disinfection dN/dt = Rate of change in the concentration of organisms with time k = inactivation rate constant, cm 2 /mW.s I = average intensity of UV light in bulk solution, mW/cm 2 N = number of microorganisms at time t t = exposure time, s Residual microorganisms protected in particles

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Components of UV Disinfection System Components of UV system Components of UV system 1. UV lamps 2. Quartz sleeves: to house and protect lamp 3. supporting structures for lamps and sleeves 4. Ballasts to supply regulated power to UV lamps 5. Power supply 6. Sleeve wiper – to clean the deposit from sleeves UV Reactors Open-Channel System Open-Channel System Closed-Channel System Closed-Channel System

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Open-Channel Disinfection System Lamp placement: horizontal and parallel to flow (a) Lamp placement: horizontal and parallel to flow (a) : vertical and perpendicular to flow (b) Flows equally divided into number of channels Flows equally divided into number of channels Each channel - two or more banks of UV lamps in series Each channel - two or more banks of UV lamps in series Each bank - number of modules (racks of UV lamps) Each bank - number of modules (racks of UV lamps) Each module: number of UV lamps (2, 4, 8, 12 or 16) Each module: number of UV lamps (2, 4, 8, 12 or 16)

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Point Source Summation a. Intensity Attenuation Dissipation: Absorption (Bears law): Divide lamp into N sections Power output of each section Intensity at a given distance from a single point source of energy: Add all point-source contributions: b. Calculation Protocol